In an in-line ultrasonic phased-array system of inspection for round bars or square bars, shear waves (shear wave) are often used to inspect the zones on and under the bar surface (subsurface), called near wall zones. For round bar inspection, usually several circular PA probes are arranged concentrically around the bar being inspected; each PA probe fires respectively clock wise (CW) and counter-clock wise (CCW) shear wave beams in an angle to the bar surface, such as 40 degrees, to inspect the surface and subsurface zones. For square bar inspection, usually flat linear PA probes are arranged in parallel to and over each side of the bar; each PA probe fires respectively CW and CCW shear wave beams of about 40 deg into the corresponding surface (for example top surface) from water couplant and the sound beams transmit to the two adjacent surfaces (for example the right and left vertical surfaces).
Before any inspections, the sensitivities of the shear wave beams have to be calibrated with one or more artificial flaws, typically subsurface side-drilled-holes (SDH) or surface notches.
Description on round bar inline inspection can be found in section 5.10.2 in <<Introduction to phased array ultrasonic technology applications>> (ISBN 0-9735933-O-X) which describes generally round bar inspection.
The calibration is not difficult to achieve for round bars, because of the revolution symmetry and rotatable nature of round bars. In more details, when a near wall SDH or a surface notch in a round calibration bar rotates with the bar around the bar axis, all focal laws can detect the same flaw and then the detection sensitivities are obtained and compensated by the software of the inspection system.
To calibrate a square bar, the above calibration method cannot be applied because a square bar doesn't have revolution symmetry.
Existing approaches in square bar in-line inspection can be found in the figure 7-105 in <<Advances in phased array ultrasonic technology applications>> (ISBN 0-9735933-4-2) which roughly describes square bar surface inspection using SW waves, the content of which is herein incorporated by reference.
Patent research for shear wave channel calibration of square bar has been carried out, but nothing similar has been found.
Similarly to the calibration practice in ultrasonic inspection of square or rectangular parts, by translating the PA probe in beam plane, the sensitivities of a shear wave channel may be calibrated with subsurface SDHs or surface notches made in the surface to inspect. In more details, a series of flaws at different depths or sound paths are made in the surface to inspect and when the PA search unit moves in probe active plane the beams can perpendicularly cross the flaws, thus allowing the sensitivities being detected. However, in the practice of in-line inspection, this calibration method is not convenient to use because, i) to achieve the translation movement of the PA probes that are arranged equally around the square bar, the movement mechanism may become very complex, ii) such mechanism is prone to introducing PA probe positioning error through backlashes of movement.